Conventionally, alternating current motors have been commonly used as a power source for various devices. In electric vehicles and hybrid vehicles, a system is typically adopted in which direct current supplied from a battery is converted into a desired alternating current by an inverter, and is then supplied to a motor. This system allows a wide range of output torques and also achieves an advantage that electrical power generated by regenerative braking can be utilized for battery charging.
Because a high voltage power supply is effective for a high power motor, electric vehicles and hybrid vehicles usually adopt a high voltage power supply on the order of several hundred volts for the main battery connected on the input side of the inverter. On the other hand, a voltage which is a half the input voltage of the inverter is typically obtained at a neutral point of a star connection motor coil. Therefore, when a battery is connected to the neutral point, two types of direct current voltages can be obtained from the system. Further, by applying chopper control to the motor coil, transfer of electrical power between two batteries can be controlled.
Accordingly, when a motor also acts as a generator in hybrid vehicles or the like, a system can be adopted in which generated electrical power is utilized for charging two batteries, thereby obtaining two power supply voltages. In particular, a capacitor can be used in place of a battery. Such a system is described, for example, in Japanese Patent Laid-Open Publication No. Hei 11-178114.
A variety of electrical equipment is installed on most vehicles, and an auxiliary battery of approximately 12V (14V when charged) is typically provided to power this equipment. Because the above-described neutral point voltage of the motor is about one half the voltage on the input side of the inverter, the neutral point voltage is significantly high in typical electric vehicles and hybrid vehicles, which makes it difficult to connect an auxiliary battery to the neutral point in these vehicles. Accordingly, a separate DCDC converter is provided for charging the auxiliary battery.
On the other hand, a so-called “dual power supply system” in which a 36-volt power supply and a 12-volt power supply are provided has been studied as a practical application example of the above system. In this dual power supply system, because it is sufficient to set the inverter input voltage to approximately 42V when charging the 36-volt power supply and set the neutral point voltage to approximately 14V when charging the 12-volt power supply, transfer of electrical power between the two power supplies can be performed using a motor coil.
Accordingly, in the above poly-phase motor driving inverter system, electrical charges can be transferred between a high voltage side battery and a low voltage side battery using a motor coil. This is advantageous in that the need for providing a DCDC converter can be eliminated.
In the inverter system for driving a poly-phase motor as described above, a large number of auxiliary equipments, commonly in the form of vehicle accessories or auxiliary devices, are connected to the low voltage side battery, and the low voltage side battery therefore serves to supply constant electrical power to these devices. Namely, in the poly-phase motor driving inverter system, switching of the switching elements in the inverter is controlled to thereby control the neutral point voltage.
However, because the neutral point voltage cannot be continuously maintained at a constant value, when the low voltage battery is not available, the voltage to be supplied to the auxiliary equipment changes significantly and constant operation of the equipment is prohibited. Accordingly, when the low voltage side battery becomes disconnected (namely, electrical connection is interrupted) in certain circumstances, the system suffers from the problem described above.
Further, in order to maintain the charging state of the low voltage side battery at a predetermined level, it is necessary to supply power in accordance with electrical power which is consumed by the auxiliary loads connected to the low voltage side battery to a power supply line to which the low voltage side battery is connected. Accordingly, conventionally, the voltage on the low voltage power supply line is sensed, and the electrical power generated by the motor is subjected to feedback control so as to maintain the sensed voltage constant.
However, the above conventional system suffers from a problem that generated power cannot be controlled in the event of that sensing of the voltage on the low voltage power supply line cannot be performed normally due to disconnection or the like, thereby causing overvoltage or low voltage.
Further, in the conventional system, a hall sensor is typically used as a position sensor and detects the position of a rotor only at 60 degree intervals. In order to obtain sufficient starting torque at the time of motor starting, 180 degree current flow type (current flows during 180 degree electrical angle and then polarity of the current is changed) is typically performed during startup of the motor. More specifically, as shown in FIG. 19, three phase rectangular wave voltage commands (indicated by a solid line, a dotted line, and a chain and dot line in the uppermost view in FIG. 19) are used to generate rectangular wave switching control signals for the inverter (gate signals Su, Sv, Sw). Accordingly, the motor driving current is rectangular wave. When a motor is a three-phase motor, 180 degree conduction gate signals Su, Sv, and Sw with 120 degree phase shift are generated for driving the motor. Here, a period in which two phases are both ON or OFF occurs for a period which is ⅙ of the current cycle. Further, for each phase current, by applying maximum current regulation which turns the corresponding phase off when the current value reaches a predetermined value, a period in which three phases are all ON or OFF occurs.
When the inverter retains the same switching state for a relatively long period and a difference between the neutral point potential and the low voltage target value is increased, the system suffers from a problem that the neutral point current changes significantly and the low voltage power supply line also changes significantly.
Japanese Patent Laid-Open Publication No. 2000-324857 describes provision of a relay between a low voltage power supply and a neutral point so that the low voltage power supply line is separated from the neutral point by the relay during startup of the motor. With this configuration, although voltage change on the low voltage power supply line during startup can be reduced, it is necessary to additionally provide a relay and to switch the relay ON and OFF as required.